This invention pertains to the field of temperature compensation. More precisely, this invention describes an indirect temperature compensation process and its embodiment in a crystal oscillator, called xe2x80x9cDigital Indirectly Compensated Crystal Oscillatorxe2x80x9d (DICXO).
Electronic clocks, and in particular crystal oscillator, are a key component of many electronics systems, such as electronic instrument, radio-communication device, data communication, electronic navigation devices, landing systems, etc. The clock frequency stability is in general a key parameter.
Besides time (aging), temperature variation is the most basic and the most important cause of clock frequency variation. In applications such as in avionics, the ambient temperature range is typically from xe2x88x9255xc2x0 C. to +85xc2x0 C. and the rate is up to 5 deg/min. In this environment, the typical uncompensated crystal oscillator drifts by +xe2x88x9250 parts-per-million, with rates up to about 3 parts-per-million/deg.
Various techniques have long been available to mitigate the temperature effects, such as the oven controlled crystal oscillator (OCXO) and the temperature compensated crystal oscillator (TCXO).
In the oven controlled crystal oscillator, the oscillator is maintained at a constant temperature in a thermally-insulated and temperature-regulated housing or oven. Unfortunately, such devices are expensive, bulky, and they need input power for the oven. Furthermore, a warming time is necessary prior working.
In the temperature compensated crystal oscillator, a temperature sensor provides feedback-control of the oscillator frequency in order to reduce the residual drift. Typically, the sensor is a hand-trimmed compensation thermistor network that feeds a xe2x80x9cvaricapxe2x80x9d oscillator tuning diode. The precise adjustment of the control circuits requires manual iterations and is of limited accuracy. Furthermore, tuning an oscillator can compromise its inherent stability; finally, the technique does not work with some types of crystal, such as the SC-cut crystals that are electrically too xe2x80x9cstiffxe2x80x9d and do not yield to electrical tuning.
An improvement of the TCXO, the digital compensation crystal oscillator (DCXO) uses built-in interpolation circuits to perform the frequency correction digitally. The calibration data are saved in a built-in memory device.
Although the average frequency can be indeed very stable, digital approximations cause a large time-jitter modulation that degrades seriously the spectral phase noise purity. Therefore, the DCXO is not suitable for most RF applications.
Besides temperature compensation, there is a need for compensating the effects of the temperature variation or thermal dynamics, due to the changes in the external environment. Large temperature variation rates can cause transient thermal distortions in various devices and can degrade the accuracy of the temperature compensation in the clock.
The usual mitigation techniques comprise, the use of an OCXO, adding thermal insulation and/or regulation to dampen the variation rates, insuring a tight thermal coupling between the compensation device and the oscillator, so as to reduce the thermal distortion.
The thermal dynamics effects depend mostly upon the clock construction, but also upon its actual installation and the thermal design of the user system. Hence there can be significant variations from installation to installation, causing unexpected performance variations. The issue of the thermal dynamics and of the actual thermal environment is particularly important for airborne GPS navigation and landing systems, which are sensitive to small clock instabilities.
The thermal dynamics from the environment will induce clock frequency variations that in turn will cause apparent errors in the user system.
For instance, at the GPS L1 carrier frequency (1.57542 GHz, 0.19M wavelength), a 1 PPM clock frequency error is equivalent to a frequency error of 1.5 KHz. This is an apparent aircraft speed error of 1500*0.19=285 M/S or 554 MPH. This error will be common to all GPS satellites being tracked, so it can be estimated by the built-in computer and cancelled out. Nevertheless, this speed error shows up and stresses the individual GPS carrier-tracking loops of the system. Similarly, the temperature variations generate higher order frequency derivatives that are equivalent to an apparent acceleration error due to the second derivative (d2F/dt2), an apparent jerk (acceleration rate) error due to the third derivative (d3F/dt3). Unless properly handled, the temperature variations can cause significant stresses on the tracking loops of the GPS navigation system that will impair its performance and safety of use. Furthermore, the apparent acceleration and jerk are critical and must be strictly controlled in particular applications such as clock aiding, where the precise value of the receiver clock time is used in the navigation solution to complement the GPS satellite measurements.
It is an object of the invention to provide a compensation indirectly in the host system without altering the subject devices, their characteristics or their outputs.
It is another object of the invention to compensate the static temperature effects and the transient as well as the dynamic effect induced at the device level.
Yet another object of the invention is to compensate the static temperature effects and the transient and the dynamic effect induced in the application at the system level.
According to one aspect of the invention there is provided a method for generating an estimation of the effects of temperature on an oscillator in a Global Positioning System (GPS), the method comprising the steps of providing a thermally coupled temperature sensor coupled to the oscillator; providing a thermal model of the oscillator; measuring the oscillator frequency at a stable temperature over a range of temperature; varying the temperature and measuring the oscillator frequency over time in order to calculate the parameters of the thermal model of the oscillator; in use, measuring a temperature using at least the temperature sensor over time; calculating the estimate temperature of the oscillator using at least said thermal model of the oscillator and the temperature value over time; outputting a signal representing the oscillator signal frequency.
According to another aspect of the invention there is provided an apparatus for estimating the effects of temperature on an oscillator in a Global Positioning System (GPS), the apparatus comprising a temperature sensor, providing a temperature signal, the temperature sensor being thermally coupled to the oscillator; an analog to digital converter, receiving the temperature signal and providing a digital temperature signal; a data storage, comprising a look-up table, the look-up table comprising a relation between a temperature and a oscillator frequency; the data storage receiving the digital temperature signal and providing an oscillator frequency; wherein the look-up table is created by using a thermal model of the oscillator with the temperature signal of the temperature sensor.